Coding of signals for transmission over a digital channel generally includes sampling an input, quantizing the samples, and forming a digital code for each quantized sample. A replica of the input is produced by decoding the digital codes corresponding to the input into a time sequence of signal samples and low pass filtering the sample sequence. As is well known in the art, some types of signals such as speech patterns are highly correlated so that portions thereof can be predicted from past values. By taking advantage of the predictive nature of such signals, the channel bit rate may be substantially reduced.
Predictive coding of signals as disclosed in U.S. Pat. No. 3,631,520, issued to B. S. Atal, Dec. 28, 1971, and assigned to the same assignee, involves the generation of predictive parameters from a succession of signal samples and the formation of a predicted value for each sample from the generated parameters and the preceding signal samples. The difference between each sample and its predicted value is quantized, encoded and sent to a receiver wherein the difference signal is decoded and combined with the corresponding predictive value formed in the receiver.
Quantizing a signal sample is accomplished as is well known by selecting the closest of a set of specified amplitude levels. The approximation introduced by quantization, however, results in noiselike distortion. Such quantization noise may be reduced by forming an error signal corresponding to the difference between the quantized and unquantized signals and modifying the signal samples in a prescribed manner responsive to the error signal. While the total quantizing noise level is unaffected by the modification, the noise may be reshaped so that it is concentrated in a specific portion of the signal spectrum where its effects are minimized. For speech signals, quantizing noise may be concentrated in formant regions of the speech signal spectrum. This results in the noise being masked so that it is not perceived by the listener.
U.S. Pat. No. 4,133,976 issued to B. S. Atal et al, Jan. 9, 1979, and assigned to the same assignee discloses a circuit adapted to redistribute quantizing noise in a speech signal spectrum during the digital encoding of the signal. The use of such a circuit in a digital coding scheme results in a significant reduction in the perceived quantizing noise. The circuit arrangements, however, are relatively complex and require that the communication system operate at a predetermined bit rate. It is an object of the invention to provide improved digital signal coding that reduces quantizing noise effects for communication at a variable bit rate.